It is advantageous in certain computer systems to provide standard interface and dimensional requirements for installing compatible devices provided by different vendors. These standards include device physical dimensions, physical connector descriptions, electrical connector descriptions, and signalling protocols for communications between devices and the central processing unit (CPU). Such standards exist and include the S100 bus, EISA bus, PC bus, AT bus, and Microchannel.TM. bus.
It is also advantageous to miniaturize a computer system for many reasons including to provide for portability or to reduce the computer's use of desk space. All of the aforementioned interfaces have considerable size requirements that limit miniaturization and portability.
One compromise solution to the size problem is to provide a small computer with a removable expansion box. Both space and an interface for standard devices can be provided in the expansion box, while keeping the main computer small.
The use of an expansion box introduces another problem. Devices internal to the computer can often be in conflict with those expansion devices in the expansion box.
Each device or memory location within a computer system has its own unique address. The central processing unit (CPU) communicates with a device by its address. Each device listens to a common bus and responds only to its own address.
If a computer containing a disk drive, for example, is plugged into an expansion box containing a disk drive and both disk drives respond to the same address, there will be a bus contention problem as both disk drives try to respond when their common address is queried. The disk drive internal to the computer can be non-standard. The internal disk drive can be blocked or readdressed to avoid contention problems. The external disk drive in the expansion box must conform to interface specifications. Blocking (deselecting) means are not provided for in any of the three standards mentioned above. The prior art does not provide separate means to execute this function. This problem is not limited to disk drives. It applies to any addressable device in a computer system. Separate selection and deselection means located in a bus interface, independent of the device being selected or deselected, are the subject of this invention.